-The entry is guided this time, meaning the landing ellipse is a lot smaller. Science wants rugged terrain with exposed bedrock; the engineers want flat plains of boring dust. More precise landing means being able to land on one and drive to the other. They've actually refined this since launch. It's smaller and closer to stuff they want to look at:

-The rover has big, beefy wheels and nuclear power. It's designed to last longer and drive farther. Here's mockups of the Sojurner rover, an MER, and the MSL:

-Landing site is called Gale Crater. There's some pretty rugged terrain surrounding the landing site. Clearly the original landing ellipse just barely fits. This site would have been impossible with previous missions:

-The rover has big, beefy wheels and nuclear power. It's designed to last longer and drive farther. Here's mockups of the Sojurner rover, an MER, and the MSL:

I always said they needed a nuclear powered Monster Truck if they were going to do robotic Martian exploration right, solar power is just not that abundant there, it's OK for electronics, but you can't get any kind of serious torque for motors with it, then dust goes and covers up the solar arrays, then winter comes, then the rover gets stuck....

Bring your own power. Thank you, and no rush with sending the consulting fee.

Just curious, how feasible would be a Mars aircraft, either fixed or rotary wing? Assuming a nuclear reactor power source, something along the lines of nuclear aircraft engines they were developing in 1950s.

I always said they needed a nuclear powered Monster Truck if they were going to do robotic Martian exploration right, solar power is just not that abundant there, it's OK for electronics, but you can't get any kind of serious torque for motors with it, then dust goes and covers up the solar arrays, then winter comes, then the rover gets stuck....

Wasn't the original design for the MERs based on RTGs, but they went with solar panels due to all the protests around the time Cassini launched?

Just curious, how feasible would be a Mars aircraft, either fixed or rotary wing? Assuming a nuclear reactor power source, something along the lines of nuclear aircraft engines they were developing in 1950s.

Isn't Mars like 38% earth gravity, while the atmosphere is 1/50th that of Earth? That would make for very unfavorable conditions for flying, since compared to earth the ratio of gravity to air density is higher. It would also make landing heavy loads difficult since there isn't much atmosphere for aerobraking.

Just curious, how feasible would be a Mars aircraft, either fixed or rotary wing? Assuming a nuclear reactor power source, something along the lines of nuclear aircraft engines they were developing in 1950s.

Unfeasible in the extreme. At the surface, Mars has a laboratory grade vacuum ("medium vacuum" by colloquial standards), 600 Pa at the mean surface level. Earth's atmosphere gives a pressure of 101,325 Pa at sea-level. The bottom of Hellas Planitia on Mars is 1,160 Pa - Whereas the summit of Mount Everest is 33,700 Pa.

Compounding this is that any craft has to carry its own oxidiser, there's no oxygen in the air, so the craft is much heavier than it'd otherwise be.

To fly in the Martian atmosphere would require an aircraft yet to be demonstrated, as it would be flying at an equivalent altitude (on Earth) of 80 km, almost in space. To fly at a distance above the Martian surface, rather than hugging it, a craft would be flying in space as defined by the Karmin Line on Earth. At this point on Earth, a craft would need to fly faster than orbital velocity to gain enough aerodynamic lift. On Mars, it would be already exceeding orbital velocity due to Mars' lower mass.

So I guess the bottom line is "you can't", since you'd be in an orbit at any altitude where you have enough forward velocity to fly.

I'm not 100% on that, and I'd very much appreciate corrections/additional data.

I hope the landing system works properly. There seems to be a great many things that could go wrong.

I admit the idea gives me the cold pricklies, but I have some confidence that it will work. After all, the airbag concept sounded silly the first time too.

The airbag concept had the advantage of simplicity. That's not a term you could use to describe the whole hovering skycrane deployment plan.

My concern, as well. Not only does the whole shebang have to survive the atmospheric entry, it has to1. Jettison heat protection2. Aim itself with ballast3. Start rockets4. Drop the package on the ropes5. Keep the rockets going6. Not smash the package on contact7. Release all of the ropes8. Fly the rocket platform away from the package

I hope the landing system works properly. There seems to be a great many things that could go wrong.

I admit the idea gives me the cold pricklies, but I have some confidence that it will work. After all, the airbag concept sounded silly the first time too.

The airbag concept had the advantage of simplicity. That's not a term you could use to describe the whole hovering skycrane deployment plan.

My concern, as well. Not only does the whole shebang have to survive the atmospheric entry, it has to1. Jettison heat protection2. Aim itself with ballast3. Start rockets4. Drop the package on the ropes5. Keep the rockets going6. Not smash the package on contact7. Release all of the ropes8. Fly the rocket platform away from the package

Ai Yai Yai

I'm uncertain as to why they chose this method over the airbag array; it seems to me that the delivery of this is exponentially more difficult than that of an airbag-protected system. O_o

I'm uncertain as to why they chose this method over the airbag array; it seems to me that the delivery of this is exponentially more difficult than that of an airbag-protected system. O_o

Mass.

Can I assume you mean that the payload has considerably more mass than we have previously delivered...and thus presumably would require unreasonably unwieldy/unworkable airbags?

Seems to me with such thin atmosphere and relatively low gravity, that makes sense...and you can get a lot more delta-V with less fuel that people tend to think of on earth. Plus, again with such low atmospheric pressure/windage, the stability of such a system will be much easier to maintain. At least at first glance.

I'm uncertain as to why they chose this method over the airbag array; it seems to me that the delivery of this is exponentially more difficult than that of an airbag-protected system. O_o

Mass.

No shit? The increased mass made airbag deployment technically not feasible? I would have thought that even the simplicity of deployment of larger airbags would have outweighed the technical complexities of this type of deployment...but I guess not!

I forget by what factor MSL exceeds airbags being practical, but it ends up being by a pretty damned huge margin. Huge enough that a complicated retrorocket system with skycrane still saves a lot of mass.

The MERs had retrorockets too, they just gave a kick to cancel most of the velocity rather than soft landed.

Soft landing isn't the end of the world. Phoenix soft landed. I was wondering whether it was a problem if the rover got swinging on the end of the tether, but apparently with the extremely thin atmosphere this is much less of a concern, controlling this is well within the control authority of the thrusters. And the rover lands wheels on ground, which avoids a risky disembarkation step.

I'm uncertain as to why they chose this method over the airbag array; it seems to me that the delivery of this is exponentially more difficult than that of an airbag-protected system. O_o

Mass.

Can I assume you mean that the payload has considerably more mass than we have previously delivered...and thus presumably would require unreasonably unwieldy/unworkable airbags?

Yes.

The MERs massed about 185 kg each. The MSL masses something like 900 kg (it's the size of a small car).

There's a point where it has to be a powered landing, and I think (no links to back this up, so I may be free-associating) the skycrane saves a non-trivial amount of mass and volume vs. a powered landing platform that the rover would roll off of. I think it was close to the limit volume-wise of what they could fit into an Atlas fairing.

Can I assume you mean that the payload has considerably more mass than we have previously delivered...and thus presumably would require unreasonably unwieldy/unworkable airbags?

Yep. Sorry about not going more in depth earlier, I was posting from my phone.

jbode pretty much covered it, and in more detail than I remembered. But from what I read of it at the time, airbags just don't scale well, and the difference between delivering the MERs and MSL was likened to the difference in delivering a golf cart vs a full on car. It's not so much that it's technically impossible, it's that an airbag solution would be practically unworkable and much more prone to failure. Airbags scale well up to MER size, but that's about the practical limit. I wish I could find a link to the original explanation I saw of it, but at the moment I'm coming up empty.

IIRC, an airbag system for MSL was ruled out for reasons of mass (primarily MSL's, but also what would have been required for the system itself), bulk, impact shock (10-20g for each bounce is much more manageable for a 180kg rover than one at 800kg), bounce height, lack of precision in final resting location, and hazard to the rover of escaping the deflated mass of airbags while trying to debark and roll onto the surface.

If I run across the link again I'll post it here, because it was pretty interesting. Until then,

I'm uncertain as to why they chose this method over the airbag array; it seems to me that the delivery of this is exponentially more difficult than that of an airbag-protected system. O_o

Mass.

No shit? The increased mass made airbag deployment technically not feasible? I would have thought that even the simplicity of deployment of larger airbags would have outweighed the technical complexities of this type of deployment...but I guess not!

A big part of ballistic coefficient is ratio of mass to cross sectional area.

I wanted to post something similar, although not as thorough as that on aerobraking.

In terms of engineering practical, the situation is even more extreme. The issue is that the airbag system consists of multiple smaller airbags. As a worst case, it has to be assumed that during impact, all weight of the landing craft will rest entirely on one single airbag cushion. And the impact velocity -> impact impulse -> impact force -> impact pressure on this one cushion will be significantly higher due to the higher weight and more poor aerobreaking of the system. I am not even sure if it possible to construct airbags that are ten times as robust as those used by the Mars Rovers.

It also had to do with the thin atmosphere not providing enough braking for the total mass as it comes in if they used airbags, hence the fact that they are using huge chutes in the current design but still needing to use rockets to slow it down further.

Everyone seems to think airbags are "easy", but the scale-up from the original Pathfinder system to MER brought with it a lot of challenges, and it's just not feasible to scale it up to something the size of MSL.

Watch the lecture, it has the best explanations of all the different trades I've seen.

That's what I thought! I wonder if some group in NASA bought a license a long time ago when nothing else was really available and using it was sort of okay . . . and it became the way things were done forever.

The description looks interesting, especially covering some of the landing ideas that aren't well known other than airbags and the sky crane. The fact that it's RealVideo, though, is one of the reasons I haven't gotten around to watching it. I also always seem to forget when I have the time in the evening.

That's what I thought! I wonder if some group in NASA bought a license a long time ago when nothing else was really available and using it was sort of okay . . . and it became the way things were done forever.

The description looks interesting, especially covering some of the landing ideas that aren't well known other than airbags and the sky crane. The fact that it's RealVideo, though, is one of the reasons I haven't gotten around to watching it. I also always seem to forget when I have the time in the evening.

I can vouch that they've been using it forever - I remember NASA stuff in RealVideo streams way back in college, if not earlier.

The Von Karmen lectures are a Caltech thing, and they are probably paid for out of the discretionary, non-NASA side of the budget..... There isn't money to go back and re-encode things every time the format of the day changes. Th new lectures are also in UStream and Flash.

Statistically there is about a 50% of success, so its basically a $2 billion coin toss.

More or less ... which is why it'd have been better to send multiple rovers. That's part of what was great about the MERs was that if one of them had been lost during landing, NASA wasn't completely screwed. For that matter, Spirit has been stuck in the sand since 2009 (and is now dead), but they're still doing stuff with Opportunity ... again the benefits of multiple rovers.

I understand that NASA only has so many resources, I'm sure that they'd have like'd have sent multiple; I guess I'm saying that it would have been better since Mars missions have had more then a few catastrophic failures and this landing procedure is extremely complex.

Really, it's enraging that the nation is too fucking cheap and stupid not to fund for dozens of rovers.